The present invention relates to injector guns for dispensing pastes, and more particularly, to injector guns that can dispense paste from a cartridge both at low pressure and high volume for filling a void and at high pressure and low volume for pressurizing the paste in the void. The present invention further includes means for connecting the injector gun to cartridges having different diameters.
Prior art injector guns have a trigger mechanism that includes a trigger in the form of a lever. The trigger includes an input end, an output end and a fulcrum between the ends. When the input end is squeezed by the user, the trigger pivots about the fulcrum causing the output end to move. The mechanical advantage of an injector gun is the amount the force applied to the input end is multiplied at the output end and can be calculated as the ratio of the length of the trigger from the fulcrum to the input end over the length of the trigger from the fulcrum to the output end. A high mechanical advantage multiplies the force more but generates less motion at the output end than does a low mechanical advantage. Therefore, a high mechanical advantage facilitates generating a high pressure in the paste but extrudes a low volume of paste whereas a low mechanical advantage generates a low pressure in the paste but extrudes a high volume of paste.
A typical application for paste injector guns is for dispensing bone cement from a cartridge into the intramedullary canal of the femur. Miller discloses such an injector gun in U.S. Pat. No. 4,338,925. Miller teaches the advantage of improved implant fixation that results from pressurizing the cement after filling the canal in order to force the cement into bony interstices. Therefore, Miller requires an injector gun with a relatively high mechanical advantage. However, as is typical of most injector guns, Miller's injector gun utilizes a trigger mechanism with a constant mechanical advantage that is a compromise between a low mechanical advantage that delivers a high flow rate for rapid filling and a high mechanical advantage that delivers high pressure for pressurizing the cement. To increase the flow of cement, the surgeon must squeeze the trigger faster. To increase the pressure on the cement, the surgeon must squeeze the trigger harder.
Some investigators have provided injector guns with user adjustable mechanical advantages. In U.S. Pat. No. 5,197,635, Chang teaches a mechanism that includes a bearing element that is adjustable up and down on the trigger and held in place by a set screw. By moving the bearing element, the output length of the trigger is changed and thus the mechanical advantage is changed. In U.S. Pat. No. 5,381,931, Chang teaches a different mechanism for selectively lengthening the output length of the trigger comprising an eccentric rotatable element attached to the output end of the trigger. Finally, in U.S. Pat. No. 5,431,654, Nic teaches a mechanism comprising two pawls attached to the trigger. The pawls are of a length and orientation such that one provides a high mechanical advantage and the other provides a low mechanical advantage. The desired pawl is engaged by means of a switch activated by the surgeon.
A disadvantage of prior art injector guns with user adjustable mechanical advantages is the need for additional parts and the resulting complexity in the trigger mechanism. Another disadvantage is the need to adjust a screw or move a switch in order to change the mechanical advantage. This adjustment typically requires both of the user's hands to effect the change. A further disadvantage of prior art cement injector guns is that they are configured to connect only to a cement cartridge having a single specified diameter. These prior art cement injector guns are therefore incapable of dispensing cement from differently sized cartridges such as from different manufacturers or different styles or sizes from the same manufacturer.